TELEPHONY NETWORKS
The telephony network allows circuit-switched paths to be established between two or more telephony users to carry voice traffic. A telephony switch, which is employed in such a network, consists of two components: (1) a call processor and (2) a switching fabric. The call processor performs signaling functions requesting connections to be established between two nodes while the switching fabric provides the physical paths or routes that allow users to send information to each other. That is, the switching fabric is a cross-connect which connects the traffic from an incoming trunk to an outgoing trunk for establishing a voice path. The activity of the switching fabric is determined by messages it receives from its local call processor. The call processor operates by sending a message to its local switching fabric instructing the switching fabric to cross-connect an incoming trunk to an outgoing trunk to develop a portion of the circuit switched path. For calls that span beyond its scope, the call processor also sends and receives messages to and from other call processors, typically in accordance with a protocol such as the ISDN User Part (ISUP) signaling protocol specified in ITU Recommendations Q.76x, described in ITU Study Group XI, Fascicle VI.8, Specifications of Signaling System No. 7, Recommendations Q.721-766, Geneva, 1988.
FIG. 1 shows an example of a circuit-switched network employing three switches 100, 101 and 102. Each switch consists of a call processor (CP) and a switching fabric (SF), as illustrated in the figure. When an originating node 200 such as a telephone sends a signaling message to destination node 201 to establish a call, a signaling message is sent to CP 100.1 via a signaling network. The signaling message includes the telephone number of the destination node 201. A data table located within CP 100.1 maps the telephone number of the destination 201 to the identifier of the next-hop switch, which is switch 101. Two activities occur in parallel as a result of this mapping procedure. CP 100.1 sending a message to its local SF 100.2 instructing SF 100.2 to connect trunk 105 (which is connected to the originating node 200) to an available trunk connected to the next hop switch 101, which in FIG. 1 is trunk 106. Additionally, CP 100.1 sends an ISUP message to the control processor of the next hop switch 101, which is denoted CP 101.1. The ISUP message informs CP 101.1 that a call request to destination node 201 is located on trunk 106.
As far as the routing of the call is concerned, the message transmitted from CP 100.1 to its local SF 100.2 includes two parameters, the incoming trunk ID, and the outgoing trunk ID. The ISUP message from CP 100.1 to CP 101.1 also includes two parameters: the incoming trunk ID (from SF 100.2 to SF 101.2), and the telephone number of the destination 201.
The above-described process is repeated in a similar manner for subsequent switches on the path. In the exemplary network of FIG. 1 the circuit switched path between originating node 200 and destination node 201 traverses three switching fabrics (SF 100, 101 and 102) to establish a circuit-switched communications path over trunks 105, 106, 107 and 108.
In summary, the establishment of a communications path over a telephony network requires the call processor of each switch to perform table look-ups, to communicate with the call processor of the next switch on the path, and to inform its local switching fabric that a particular pair of trunks should be connected.